1. Field of the Invention
The present invention relates to a switching control circuit and a switching power supply circuit.
2. Description of the Related Art
A ripple converter is known as a switching power supply circuit that generates an output voltage of a target level out of an input voltage (see, e.g., Japanese Laid-Open Patent Publication No. 2006-14559). FIG. 6 is a diagram illustrating an example of a common ripple converter 100 that employs a fixed-on-time system. A control circuit 110 turns on an NMOS transistor 120 to be kept on for a predetermined time period, when a feedback voltage Vfb that corresponds to an output voltage Vout becomes lower than a reference voltage Vref. As a result, the output voltage Vout is increased. When the output voltage Vout is reduced due to the influence of a load 150 and the feedback voltage Vfb becomes lower than the reference voltage Vref, the control circuit 110 again turns on the NMOS transistor 120 to be kept on for the predetermined time period. These operations are repeated, so that the output voltage Vout of a target level is generated.
The output voltage Vout of the ripple converter 100 is more mildly reduced as the load current of the load 150 becomes smaller, and thus the switching period of the NMOS transistor 120 becomes longer. In other words, the switching frequency of the ripple converter 100 is reduced when the load current becomes smaller. Therefore, when the load current is reduced and the switching frequency is reduced to a frequency in the audible range, for example, noise may be generated from the ripple converter 100. Therefore, for example, the control circuit 110 is designed so as not to cause the switching frequency to become lower than or equal to a predetermined frequency that is higher than the audible range.
When the control circuit 110 as described above is used, the switching frequency never becomes lower than or equal to the predetermined frequency, thereby suppressing the generation of noise, for example. However, even in the case where the output voltage Vout is not reduced substantially, the control circuit 110 may turn on the NMOS transistor 120, thereby causing a problem that the output voltage Vout is increased from the target level.